Field of Invention
This invention relates in general to pumps and in particular to vacuum pumps.
Background Description of Prior Art
Over the past decade, especially since the terrorist attacks of 9/11, there has been an increasing demand for sensitive chemical, biological, and explosive detection devices. The vast majority of these detection devices have presented challenges to the scientists and design engineers to translate what has previously been the domain of the laboratory, into small hand-held portable devices. Of the family of detectors most widely used to meet the challenge, none has greater potential, yet been more difficult to miniaturize into a portable form factor than the mass spectrometer. Mass spectrometers, unlike ion mobility (IMS), require a partial pressure region to scan for a given mass number indicative of the trace species of interest. Probably the most significant hurdle yet to overcome is how to create a small, cost effective, low power vacuum system. Over the past 30+ years, no significant advance in vacuum pump concepts save for the turbomolecular pump, have been realized. The disclosed invention offers a potential for game-changing new technology that could make the turbo pump obsolete in many applications while promising to provide significant cost savings with unprecedented reliability and longevity.
Mass spectrometers necessitate a partial pressure zone to allow for the process of mass characterization of the analyte under consideration. For proper operation of a mass spectrometer, the normally neutral molecules must first be changed into charged ions before attempting this characterization. By transforming neutral molecules into charged molecules, one now has the ability to control the trajectories and destinations of charged species by appropriate combinations of electric and magnetic fields. In addition, detectability is generally much greater for ions than for neutral molecules because each ion gives rise to at least one electron in a primary signal current that can be greatly amplified by well known techniques. Moreover, ion-electron multipliers can produce millions of electrons in signal current for each incident ion. It is true that some optical techniques, e.g. laser induced fluorescence can sometimes provide larger signal/molecule ratios than can ion-multiplication techniques but the conditions required for such sensitivity are often more difficult to achieve, vary much more from species to species, and require more expensive gear than is generally the case with ions. The target background pressure for most mass spectrometers is generally 1×10−6 Torr. Up until about 20-30 years ago, most vacuum system pumping was performed by a vapor-jet pump, sometimes referred to as an oil diffusion pump. Both terms are correct, in that a heated oil of limited vapor pressure with an oil molecular mass at least 17 times that of air, is configured to produce a high velocity oil vapor jet into which air molecules diffuse and thus undergo collisions with the high translational energy of the oil jet. Compression of the air by repeated oil molecular collisions drives the air molecules onto the pump walls which are cooled and results in condensation of the oil and entrained air species. A fore pump exhausts the residual air gas and the oil is re-heated so the process can repeat itself. In practice, it generally takes about 5 oil molecules to pump one molecule of air. While diffusion pumps have proven themselves capable of removing a vast gas load—far more than equivalent sized turbomolecular pumps—there are significant drawbacks. First of all, oil diffusion pumps are not “instant-on” devices like a turbo pump. It can take many hours before a diffusion pump heats up enough before sufficient pumping is realized. Next, diffusion pumps are notoriously inefficient power wise, having an efficiency of only about 1%. Of all the power used by the electric heaters to heat the oil, most of this energy ends up as waste via the cooling coils used for oil condensation. Because of the heating required to heat the oil, a diffusion pump can't be readily shut down either. If one needed access to the partial pressure region of a mass spectrometer, a gate valve would be needed to isolate the diffusion pump, lest explosive flash of the oil and subsequent chamber contamination with oil would transpire. Finally, diffusion pumps are by their nature axis sensitive. Tilting a diffusion pump risks exposing the heater and burning it out or dispensing the hot oil into the vacuum chamber. In the least worst-case scenario, the pump would simply cease to function if the oil supply were interrupted by a change in orientation. In spite of all these drawbacks the diffusion pumps are still widely used due to their great pumping speed, simplicity, reliability (many operate continuously for years at a time), and low cost
Given all of the limitations of ordinary oil diffusion pumps, it is not surprising that the development of the turbomolecular pump was welcomed in the vacuum industry. The turbomolecular pump is very much like the compressor section of a jet engine. Marsbed Hablanian, one of the leading scientists in vacuum technology today, describes the evolution of the turbo pump as follows. “Turbomolecular pumps are essentially axial-flow compressors designed for pumping rarefied gases. Original designers adapted more or less traditional axial-flow compressor stage arrangements using mathematical modeling that was based on studies of molecular trajectories inside alternating rotating and stationary blade rows. Recent design trends lean toward hybrid stage arrangements, which incorporate turbomolecular and turbodrag stages within the same body and mounted on the same shaft. The new pumps achieve much higher compression ratios (10 to 100 times), permitting higher discharge pressures and allowing the use of oil-free backing pumps. When engineers first encounter high-vacuum technology, they sometimes have difficulty in understanding molecular flow concepts but, after a few years, they seem to have more difficulty in associating the rarefied gas flow with the more common higher pressure viscous flow. There are similarities, however, between the two and the appreciation of such similarities can lead to better designs. High-vacuum pump technology was developed mostly by experimental physicists, electrical engineers, and some chemists. As a result, unique design concepts developed and the descriptive terminology is not well-related to mechanical engineering and, specifically, to fluid mechanics. Even worse, because different vacuum pumps have been developed by different persons in different times, the terminology used to describe the performance of various pumps is different. For example, even though turbomolecular and vapor jet pumps are very similar in their basic performance, the compression ratio values are firmly associated with turbomolecular pumps but almost never mentioned in relation to vapor jet pumps.”
When the pressure is high (near atmospheric) and the rotor velocity is low, the pumping action will be extremely inefficient. If any pressure difference is established due to drag at the surface, it will be lost immediately because of backflow some distance from the rotor. But under molecular flow conditions and the peripheral rotor velocity approaching the average velocity of gas molecules, a significant pressure difference can be maintained. Every collision with the rotor will send the molecule back to the discharge area. The disk type has the disadvantage of lower tangential velocity in the spiral grooves near the center, but is easier to balance and has a single rotor. The pumping speed of early pumps was low. The art of making high-speed rotating machinery had not yet been developed (modern turbos spin typically from 40,000 RPM to over 200,000 RPM—the smaller the pump, the faster the rotor speeds). Modern turbomolecular pumps of open, thin bladed, axial-flow type appeared in the early sixties. An axial-flow compressor consists of a set of alternating bladed rotors and stators. The problem with turbomolecular pumps is the complexity and cost of manufacture, and some potential safety issues especially with regards to portable applications. The tolerances are extremely critical for both rotors and stators. In addition, life issues of the bearings can impact any portable design, and as a turbo pump often represents the highest cost item in any spectrometer system, replacement essentially requires scraping of a portable mass spectrometer due to the high cost of a new turbo. Some of the smallest turbos, from 5 to 70 liters/sec, have an average price ranging from $5,000-$10,000 depending on the quantity purchased by an original equipment manufacturer (OEM). In addition, as the size of a turbomolecular pump is reduced, the rotor speed must be increased to have the tip velocity match the thermal velocity of the gas to be pumped. The loads thus induced can make for rather spectacular system failures throwing shrapnel everywhere.
Another issue for small handheld instruments is the effect due to torque-induced gyroscopic precession. Given the extremely high rotational rotor speeds in miniature turbos, any sudden movement or jarring by the user of an instrument would immediately be accompanied by a gyroscopic “pitching” of the device. In a severe case, if the inertia of the rotor were high enough, the handheld unit could be twisted out of the user's hands and dropped. Dropping a portable mass spectrometer is bad enough, but with an internal turbo spinning at over 200,000 RPM, shaft/blade flexure could cause a turbo explosion, sending shrapnel inside the device. Even if an instrument were not dropped but accidentally hit a hard surface briefly such as a table, the transient force could be many thousands of g's for a millisecond—enough to crash a spinning turbo. In this sense, walking around with a handheld device with a running turbomolecular pump would be akin to walking around with a grenade with the pin ready to fall out!
Prior research by the inventors and others using electrosprays for use in micro-satellite propulsion, also known as “colloidal” propulsion, has demonstrated that high velocity jets of charged droplets can provide thrust due to momentum transfer. The key inventive element in the disclosed invention is the realization that the same electrospray micro-satellite propulsion research can be adapted into creating a new type of vacuum diffusion pump. This electrospray pump, instead of using heated oil vapor to interact with molecular gas flow in a vacuum system, employs the electrospray jet to accomplish the same feat, albeit without the added power expense of heating oil as in a contemporary oil diffusion pump. What is currently being done for colloidal propulsion parallels the requirements for an effective vapor-jet pump. The attractive feature of colloidal droplets produced by the electrospray phenomena is significant. Principal among these is the lack of volatility of the working fluid (as a volatile propellant would evaporate in the vacuum of space), no need for diffusion pump heater concepts, and the ability to produce ions or droplets at a known velocity that exceed the thermal velocity of target pump gases while only using milliwatts of power.